Odd discovery may help refine theories about how planets form

Jun 17, 2009

An international team of researchers has found a planet around another star whose orbit is steeply tilted from the plane of the star's equator, a finding that contradicts some theories about how solar systems form.

In our own solar system, all of the planets orbit the sun almost exactly in the same plane as the sun's rotation - and that alignment is required by currently accepted theories of how stars and planets form from a collapsing disk of dust and gas. Any misalignment, such as the one the team found, must have occurred as a result of a disturbance sometime after the planet's formation, theorists say.

Astronomers are interested in exploring the characteristics of such distant planets partly to help refine theories of planet formation, and partly just to understand the kinds of variations that may be possible in the universe around us - to "see how the dice get rolled in other solar systems," says MIT physicist Joshua Winn, who led the team that measured the planet's tilted orbit.

Detecting this oddball orbit required a combination of good luck, advanced technology and ingenious methodology. Winn, assistant professor of physics in MIT's Kavli Institute for Astrophysics and Space Research, and a team of astronomers used one of the world's two largest telescopes to make the painstaking observations that confirmed earlier hints of this planet's unique orbit.

The planet, called XO-3b, was discovered in 2007 through a method that depends on a chance alignment of the planet's orbit with the line-of-sight between its star and the Earth. Because of that alignment, the planet sometimes passes directly in front of the star as seen from here - an event called a transit - thus causing a slight dimming of the star's light. That dimming can be detected with a powerful telescope connected to a highly sensitive light meter, or photometer. Of the more than 350 exoplanets discovered so far, fewer than two dozen have been found through this transit method.

Detecting the planet itself was relatively easy, as it dimmed the star's light by about 1 percent. But to go one step further and measure the angle of its orbit, even with such powerful tools, means that "we have to be sneaky about it," Winn says. It turns out that if a planet crosses the star's disk at an angle to the star's own rotation, it causes a distinctive pattern of change in the overall color of the star, as measured by a highly sensitive spectrograph, because of the Doppler shifts caused by the star's rotation.

Hints of such a spectral signature were seen last year by another team, but that team acknowledged that they could not be confident of their result. The new observations, carried out by Winn and his team in February at the Keck I Observatory in Hawaii, provided a clear, solid measurement of the planet's distinctive tilt, determining the angle of the orbit to be about 37 degrees from the star's equator. The results are reported in a paper in the Astrophysical Journal, which was recently posted online and will be published in the journal's August issue.

A majority of the planets discovered so far orbiting other stars - known as exoplanets - are very large planets comparable to the gas giants in our solar system, but orbiting their stars much closer in (and thus faster). That's because the method used to detect these planets makes it much easier to detect such close-in giants than smaller or more distant ones. In the case of XO-3b, it is about 13 times as massive as Jupiter, yet orbits its star with a period, or "year," of just 3.5 days (Jupiter, by contrast, takes almost 12 years for an orbit). That size and closeness to its star are "unusual, even by the standards of exoplanets," Winn says.

Such "hot Jupiters" - so named because they resemble the solar system's largest planet, but would be much hotter because of their proximity to their parent stars - could not have formed in the places they are seen now, according to accepted planet-formation theory. They must have formed much further out from the star, then migrated inward to their present positions. Astronomers have come up with different mechanisms to account for the migration: the gravitational attraction of other planets as they passed close by, or the attraction of the disk of dust and gas from which the star and its planets formed.

Close encounters with other planets could greatly amplify a slight initial tilt, but attraction from the disk of material could not. So that theory could not account for a planet ending up on such a tilted orbit, which rules out that theory at least in the case of this particular planet.

In coming years, as new telescopes such as the Kepler space observatory begin to discover increasing numbers of exoplanets, "it will be interesting to identify more that are tilted, to find enough of them to be able to tease out patterns," Winn says.

In addition to Winn, the team included John Asher Johnson of the University of Hawaii; Daniel Fabrycky, Gil Esquerdo and Matthew Holman of the Harvard-Smithsonian Center for Astrophysics; Andrew Howard and Geoffrey Marcy of the University of California, Berkeley; Norio Narita of the National Observatory of Japan; Ian Crossfield of UCLA; Yasushi Suto of the University of Tokyo; and Edwin Turner of Princeton University. The work was funded by the NASA Origins program, an NSF postdoctoral fellowship and World Premier International Research Center Initiative.

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User comments : 9

solar system formation be damned...i think this has higher level implications then that within astronomy and physics.

Correct me if I'm wrong here, but based on the concept that gravity is basically a dip, or trench if you will, in the fabric of space time caused by the weight of any given object (but speaking to planets here of course)...with me so far...so if that is the case, like all dips or trenches in a fabric caused by the weight of an object, its strongest elliptical plane (or orbital point), if you will, should always be approximately at the equatorial area with it being the bulkiest/heaviest part of a planet (but a number of 10s of thousands of miles out from the planet surface of course to allow for "bending room" of the fabric (put a pool ball on a shirt youre holding firmly, the dips circle will be further out than the pool balls actual diameter). This effectively means to get over the gravity caused by any given mass, you must exit the dip or trench it creates in the fabric...
Sound right so far, or rather, are ya trackin on what I'm tryin to say so far?

So now my first ? on this is:
If the orbit of a planet is way out of alignment with the equatrial plane of its host star, wouldnt this completely and undeniably invalidate that theory of the cause of gravity (being the dips/trenches caused by the weight of matter)?

Unless the answer to my second question turns out true, which is:
Is it possible that this indicates that the fabric of space is twisted, and can be in different directions, potentially changing the location of the dip? (of course, that would entail that the fabric itself is pulling upon the planets to essentially pull them in so they make a dip.

In theory, if you had a ridiculously sensitive instrument, you could detect the doppler shift from one side of the star to the other side of the star. So that you detect shorter wavelengths coming from the side that is rotating towards you, and longer wavelengths coming from the side that is rotating away from you, and medium wavelengths at the star's poles. But this would only work if the Poles were pointed at a direction nearly perpendicular to the observer's line of sight.

As for what method the astronomer's actually used, and whether they have anything sensitive enough to perform the technique I suggest, I have no idea.

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This is clearly sticking a fork in the standard model of solar system formation, in any case.

In the standard model, the higher the angle of the planet's orbit, the younger you would expect it to be, because as time goes on, the orbits of plantes should approach the equator of the star as their gravity smooths one another out to approximately the same plane. So the orbit of this planet suggests it should be very young.

However, The mass of this planet suggests it should be very old, as it would logically take more time to collect more matter.

13 jupiters masses is basically more mass than that of all objects in our solar system combined except the sun itself, and this would include all of the comets and all the dwarf planets.

It seems more likely that this is a failed rogue star which has been capture by another star, rather than anything which formed in any sort of accretion disk environment.

Quantum_Conundrum: That is one of the methods used to detect the rotation rates of stars. Our instruments are more than sensitive enough to detect such a Doppler Shift. However, there is a great deal of error in that. Not in the measurement itself, but because you don't know how the star's poles are aligned with respect to you. If they are completely perpendicular to Earth then you can take the result at face value. Same if they are parallel. So the end result ends up with some pretty large error bars on it.

But it still gives you a definite maximum or minimum value for rotational rate.

It occurs to me, all of our conversations thus far may be null and void, as they do not mention if any other known planets are in this solar system. if we can discover another, and see its orbit, then it would be known if that ones is truly oddball, or if that system is just on an angle, so to speak....

Even if it is an oddball planet, this article jumps to an odd conclusion. It's suggesting that this planet needed to have formed this way, and that goes contrary to models of planetary formation. Well, that would be true, if the planet *needed* to have formed this way in order for its orbit to be explained.

By far and away the most likely explanation is simply that there was a three-body gravitational interaction. IE, 2 planets, and the star, or the planet, its star, and a close brush with another system, or a rogue blackhole, or a neutron star, or any number of things.

So it's not like this is unexplainable or anything. I actually don't understand the point of the article, since the entire premise of the argument it makes is flawed.